There is a requirement in industry for the measurement of conditions such as strain or temperature at all points over long distances. Typical uses are for monitoring oil and gas wells, long cables and pipelines. Distributed temperature sensors often use Raman or Brillouin components of scattered light in optical fibres as the means to determine the temperature. Here, light from an optical source is launched into a fibre and the small amount of light that is scattered back towards the source is analysed. By using pulsed light and measuring the returning signal as a function of time, the backscattered light can be correlated to distance along the fibre. This backscattered light contains a component which is elastically scattered (Rayleigh light) and components that are up- and down-shifted in frequency from the source light (Raman and Brillouin anti-Stokes and Stokes light respectively, also known as inelastic scattered light). The powers of the returning Raman components are temperature dependent and so analysis of these components yields the temperature. The powers and frequency of the returning Brillouin components are strain and temperature dependent and so analysis of both components can yield temperature and strain independently.
The principles of analysing Brillouin backscatter for measuring strain and temperature has been described before, and reference is made to:
Parker, T. R., Farhadiroushan, M., Handerek, V. A., and Rogers, A. J., “Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibres”, Optics Letters, 1 Jun. 1997, Vol. 22, No. 11, pp. 787-789 and to:
Parker, T. R., Farhadiroushan, M., Feced, R., Handerek, V. A., Rogers, A. J., “Simultaneous Distributed Measurement of Strain and Temperature from. Noise-initiated Brillouin Scattering in Optical Fibers”, IEEE Journal of Quantum Electronics, April 1998, Vol. 34, No. 4, pp. 645-659.
If the frequency and power of the Brillouin backscatter can be measured then the strain and temperature in the fibre can be determined. Measuring the frequency of the backscattered light has required frequency analysis, by sweeping of a source wavelength, or a filter across the backscattered signal. From the profile of amplitudes at different wavelengths, a peak can be determined and the frequency of the peak, or the average frequency of the profile determined. It is known that the frequency analysis can be conducted in either the optical or electrical domain. The sweeping process can take some time especially since the signal under study is extremely noisy in nature.
As the fibre length increases, the accuracy of the temperature and strain measurements reduces. One reason is because the overall loss over the length of the fibre increases and so the signal returning from the far end is smaller and, as a consequence, noisier. One way to reduce the errors caused by noise is to take an average of many measurements using many pulses. The amount of such averaging can be limited by the maximum pulse repetition rate. This rate is usually limited by the fact that it is normally only possible to usefully have one pulse in the fibre at any time as otherwise it would not be possible to determine where the returning signal was generated (the backscattered signals from the multiple pulses would overlap). As the time for a light pulse to travel along a fibre is proportional to the length of the fibre thus the maximum pulse repetition rate decreases as the length of the fibre is increased. Hence accuracy drops as the sensing length is increased. As discussed above, one measurement of frequency may need many pulses, (typically hundreds) and so the effective measurement repetition rate is many times lower (perhaps hundreds of times lower) than the maximum pulse repetition rate.
An example of this is shown in U.S. Pat. No. 6,380,534, assigned to Sensornet. A narrow band Fabry Perot filter is swept across the spectrum to pass different wavelengths at different times to provide a profile of power at different wavelengths. The optical filter is scanned at a slower rate compared to the pulse repetition rate of the source. This allows the backscattered light to be captured at different selected wavelengths by sending many optical pulses during one scan cycle. The backscattered traces are combined to construct the full spectral response of the backscattered light along the sensing fibre. Each spectral response, corresponding to a different section of fibre, is normalised with reference to its Rayleigh peak which is insensitive to temperature and strain. The measurements are calibrated with respect to the spectral response of a reference section, of known temperature and strain, and the strain and temperature distribution along the sensing fibre are computed by measuring the relative amplitude and position of Brillouin peaks.
Another known example shown in a paper by Kee, H. H., Lees, G. P., and Newson, T. P., “Low loss, low cost spontaneous Brillouin-based system for simultaneous distributed strain and temperature sensing”, CLEO 2000, CTh14, San Fransisco, May 2000, uses an interferometer for distinguishing the Brillouin component, and a second interferometer for determining the frequency. This uses a two step process using two pulses at different optical source wavelengths to calculate a single frequency value. This is then repeated to enable averaging. A disadvantage of this is the time required to change the optical source wavelength between different positions. This in turn can introduce frequency errors depending on the accuracy and repeatability of the optical source wavelength tuning. Another known example shown in Muaghan, S. M., Kee, H. H., and Newson, T. P., “Simultaneous distributed fibre temperature and strain sensor using microwave coherent detection of spontaneous Brillouin backscatter”, Measurement Science and Technology, Vol. 12, 2001, pp. 834-842, uses coherent detection to convert to the electrical domain. The electrical signal is analysed using an electrical spectrum analyser to determine the frequency of the Brillouin components. The electrical spectrum analyser relies on mixing the received optical signal with a local RF oscillator, and sweeping the frequency of this oscillator to determine the spectrum of the received signal. Once again this method requires frequency sweeping of a source, this time an RF oscillator, which reduces the achievable measurement speed.